This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-105640 filed on Apr. 13, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a device for driving a capacitive load such as an electroluminescent panel.
2. Description of Related Art
A circuit for driving an electroluminescent panel is disclosed in JP-A-9-305144. This driving circuit includes a circuit for controlling a charging and discharging current of capacitive electroluminescent elements at a constant level (this circuit is referred to as a constant-current circuit). The constant-current circuit prevents an impulse current otherwise supplied to the electroluminescent elements. More particularly, as shown in FIG. 9A attached to this specification, the driving circuit includes a charging circuit 1 and a discharging circuit 2. When an FET 1a (a field effect transistor) in the charging circuit 1 turns on according to a control signal-1 fed through a coupling condenser 1c and an inverter 1d, an electroluminescent element 3 is charged with a voltage VH+ through a resistor 1b and the FET 1a. On the other hand, when an EFT 2a in the discharging circuit 2 is turned on according to a control signal-2 fed through a coupling condenser 2c, the electroluminescent element 3 is discharged through the FET 2a/and a resistor 2b. 
As shown in FIG. 9B, the charging current Iel in the above-described circuit abruptly rises upon turning-on of the EFT 1a, and the discharging current Iel abruptly flows upon turning-on of the FET 2a. A terminal voltage of the electroluminescent element 3 linearly rises up to the voltage level of VH+, the linearly drops down to the level of 0 volt. This means an impulse current flows upon turning-on of the FETS 1a and 2a. The impulse current generates radio noises though the constant-current circuit controls the charging and discharging current at a constant level after it reaches that level. If the resistors 1b and 2b are eliminated as shown in FIG. 10A, the charging and discharging current change further abruptly as shown in FIG. 10B. Accordingly, further higher radio noises are generated by the impulse current.
To cope with the problem described above, JP-A-2-256191 (the counter part: U.S. Pat. No. 5,027,040) proposes to insert a coil between the FET 1a and the electroluminescent element 3 and another coil between the FET 2a and the electroluminescent element 3. Since the coils alleviate sharp rising-up of the impulse current, the radio noises will be reduced. However, coils having large inductance are required to reduce the radio noises in the proposed manner for the following reason. A frequency fel for driving the electroluminescent element in the circuit shown in the above JP-A-2-256191 is determined according to the following formula: fel(Hz)=1/{4xcfx80(LCel)xc2xd}, where L is an inductance of the inserted coil, and Cel, is a capacitance of the electroluminescent element. If Cel is 2 (nF), and fel is 400 (Hz), L has to be in the order of 20 (H) to effectively suppress the impulse current. It is, however, difficult to buy such a coil having a large inductance, because such a coil is not available in the usual market.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved device for driving a capacitive load such as an electroluminescent panel, in which radio noises are effectively suppressed without using coils having a large inductance.
A driving device of the present invention alternately charges and discharges a capacitive load. The driving device is composed of a charging circuit connected between a power source and the capacitive load and a discharging circuit connected between the capacitive load and a ground. The charging circuit is a series circuit including an inductive coil and a transistor that is turned on or off according to a control signal fed thereto. The charging circuit constitutes a series L-C circuit together with the capacitive load. The discharging circuit is a similar series circuit including an inductive coil and a transistor that is turned on or off according to another control signal fed thereto. The discharging circuit also constitutes a series L-C circuit together with the capacitive load.
The capacitive load is charged through the charging circuit upon closing the charging circuit, while it is discharged through the discharging circuit. An impulse current that otherwise flows into the capacitive load upon turning-on of the transistor in the charging circuit is suppressed by the inductive coil. Similarly, an impulse current otherwise flowing out of the capacitive load upon turning-on of the transistor in the discharging circuit is suppressed by the inductive coil. Since the impulse current flowing in and flowing out of the capacitive load is suppressed, radio noises generated by the impulse current are prevented. A resistor may be additionally inserted in both the charging and discharging circuits thereby to limit the charging and discharging current to a constant level, while suppressing the impulse current by the inductive coil.
An electroluminescent display panel having capacitive pixels may be driven by the driving device of the present invention. The electroluminescent panel is composed of an array of scanning electrodes, an array of data electrodes and electroluminescent layer interposed between both arrays. Pixels are formed at each inter section of both electrodes, and they are alternately charged and discharged by the driving device to selectively activate the pixels. Since the pixels are capacitive elements, they are driven by the driving device of the present invention in the same manner as other inductive loads. The impulse current in charging and discharging is suppressed, and thereby radio noises are prevented.
A diode may be connected in parallel to the inductive coil in the charging circuit to limit a level of the charging voltage imposed on the capacitive load. To supply scanning voltages sequentially and selectively to the scanning electrodes of the electroluminescent panel a thyristor may be connected between the driving device and each scanning electrode.